1. Field of the Invention
The present invention relates generally to hybrid electric vehicles (HEVs), and specifically to a method and system to improve the efficiency and drivability of a HEV by monitoring vehicle control variables and their rate of change, whereby driver demand is anticipated so that unpredicted or undesired engine false starts and performance lags are prevented.
2. Background Art
The need to reduce fossil fuel consumption and undesirable engine exhaust gas emissions from vehicles powered by an internal combustion engine (ICE) is well known. Vehicles powered by battery-powered electric traction motors have attempted to address this need. However, electric vehicles have limited operating range and limited power capabilities. They also require substantial time to recharge their batteries. An alternative solution is to combine an ICE and an electric traction motor in one vehicle. Such vehicles are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 (Severinsky). HEVs reduce both undesirable exhaust gas emissions and fuel consumption because a smaller engine can be used. Under certain conditions, the engine can be turned off.
The HEV has been described in a variety of configurations. Many known HEV designs use systems in which an operator is required to select between electric and internal combustion engine operation. In other configurations, the electric motor drives one set of wheels, and the ICE drives a different set of wheels.
Other, more useful, configurations include, for example, a series hybrid electric vehicle (SHEV), which is a vehicle with an engine (most typically an ICE) that powers a generator. The generator, in turn, provides electric power for a battery and an electric traction motor coupled to the drive wheels of the vehicle. No mechanical connection exists between the engine and the drive wheels. Another useful configuration is a parallel hybrid electrical vehicle (PHEV), which is a vehicle with an engine (most typically an ICE), battery, and electric traction motor that combine to provide torque to the drive wheels of the vehicle.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both the PHEV and the SHEV. The PSHEV is also known as a torque (or power) split powertrain configuration. In the PSHEV, the engine torque can be used to power a motor/generator and/or contribute to the necessary traction wheel or output shaft torque. The motor/generator generates electrical power for the battery, or it can act as a traction motor to contribute to the necessary wheel or output shaft torque. The traction motor/generator can be used also to recover braking energy to the battery if a regenerative braking system is used.
The desirability of combining the ICE with an electric motor/generator is clear. Fuel consumption and undesirable engine exhaust gas emissions are reduced with no appreciable loss of performance or range of the vehicle. Nevertheless, there remains substantial room for development of ways to optimize HEV operation. This includes the need to ensure that vehicle drivability is consistent, predictable and pleasing to the customer while also maintaining efficiency.
Factors involved in achieving an acceptable level of HEV drivability are the frequency and character of engine-start and engine-stop events. Frequent engine starts and stops can be annoying, especially if they do not occur in response to any conscious input from the vehicle driver.
Some engine starts and stops are dictated by an energy management strategy (EMS) that seeks to combine the engine and motor drives to achieve maximum fuel economy. For example, the EMS might start the engine whenever demand exceeds a predetermined motive power threshold. Also, the engine must start when driver demand for power is in excess of that available from the electric system.
Frequent, annoying, high-emission, and engine-wearing “false starts” can occur when the engine is started in response to what later proves to be a very brief demand for power in excess of the motive power threshold but still within the drive capabilities. This can occur when quickly pulling out into otherwise slow traffic or surging forward in heavy traffic. Alternatively, starting an engine poses a challenge because its torque is not available instantaneously. An annoying lag in performance will occur if the engine is not started somewhat in advance of the actual engine torque requirement.
An HEV system controller (VSC) must, therefore, control two mode transitions. The first is the transition from a vehicle at rest with the engine off to a vehicle using electric power. The second is the transition from electric driving to engine power in response to an increase in driver demand. (This driver demand transition should not be confused with a less time critical version of the same transition when the engine is started because of a need to charge a battery.) The timely preparation for these transitions is achieved by “anticipators”.
A converterless multiple ratio automatic transmission of the kind that may be used in a parallel hybrid electric vehicle powertrain is shown in U.S. Pat. No. 6,217,479, where an engine crankshaft is connected through a damper assembly and a disconnect clutch to the torque input element of multiple-ratio gearing without an intervening torque converter. A slipping forward-drive clutch during an engine-engage operating mode is used, thereby avoiding a need for a separate startup clutch. The lack of a startup clutch, as well as the lack of a hydrokinetic torque converter, reduces the inertia mass which permits a faster response to a command force startup torque at the vehicle wheels.
A control strategy for a hybrid powertrain of the kind disclosed in the '479 patent is described in U.S. Pat. No. 6,364,807. The control strategy of the '807 patent includes a closed-loop clutch pressure modulation technique to effect a smooth transition from an electric motor drive mode to an internal combustion engine drive mode. This is done in cooperation with a control of the fuel supply during the transition. The electric motor in this HEV powertrain may act as an inertial starter, wherein the electric motor freely accelerates up to idle speed where the engine-driven pump has full hydraulic pressure for the clutch following continuously slipping clutch operation during startup.
Another hybrid vehicle powertrain using a multiple-ratio transmission without a torque converter and having a startup clutch located between the induction motor and the engine crankshaft is described in U.S. Pat. No. 6,176,808. An auxiliary launch torque is supplied by the motor during startup in the design of the '808 patent, and regenerative braking with the internal combustion engine inactive is available for charging the battery when the vehicle is in a coast mode.
In HEV operating strategies of the kind described in these prior art patents, the decision to start the engine in response to driver demand is based on vehicle speed and driving torque. The drive power is calculated using torque and motor speed. The total power required for the HEV is not only the drive power, but also power for all other loads, such as accessory load and climate control load. If this total required power exceeds a predetermined threshold for the motor, engine power and, therefore, engine start is required. If the total required power is below a predetermined value, the motor solely provides torque to the powertrain. A hysteresis loop is included in these predetermined values to prevent mode “chattering” when the vehicle nears these power thresholds.
A problem with this prior art system is apparent in an intermediate power range above the power below which it is more efficient to drive with the motor on (perhaps five to ten kW for a typical compact to mid-size vehicle), and below the peak electric-only power capability required for acceptable engine-off launch (twenty to forty kW for the same vehicle). While driving in electric-only mode, a momentary demand for power in this intermediate power range should be met without repetitive starting the engine, and then immediately stopping it. Therefore, a new anticipator strategy is required to improve efficiency and drivability of the HEV by anticipating the need for a driving state or mode change as close as possible to a predetermined optimal moment to create a seamless transition while reducing or eliminating engine “false starts”.
It is possible to effect an instantaneous response of the powertrain to a driver command using a so-called feed forward torque calculation. This strategy would anticipate the torque requirements following a command for an increased torque or a decreased torque by calculating a leading indicator of engine torque. That indicator is used to develop a transmission line pressure that is appropriate for a subsequent ratio change and a subsequent driving torque requirement by anticipating the engine torque required following a response to an engine torque request by the driver. This torque feed-forward technique is further refined in a control system described in U.S. Pat. No. 6,253,140 where the engagement of the clutch, as the gear ratio change nears completion, is controlled with a pressure-shaping function used to reduce the desired slip rate to effect a smooth termination of the slip of one friction element as a companion friction element during a ratio change gains torque-transmitting capacity. This is achieved using an adaptive engagement technique so that the engagement characteristics of the controller can be learned during a ratio change and used in a subsequent ratio change. In this way, ratio change smoothness can be achieved by compensating for driveline variables such as changes in coefficients of friction due to clutch wear, for example, and due to changes in spring loads at the friction element actuators.
Another example of a converterless multiple-ratio transmission is described in U.S. Pat. No. 6,299,565 wherein the clutches, during a ratio change, are controlled by a strategy that makes it possible to achieve maximum vehicle acceleration using a controllable wet clutch between the engine and the input of a synchronous transmission.
Another example of a hybrid electric vehicle powertrain is described in U.S. Pat. No. 6,316,904 wherein the induction motor is controlled using a speed sensorless controller.
The control systems of these prior art patents do not describe nor suggest an anticipator function for anticipating the need for a driving state change so that an optimal seamless transition between driving state modes can be achieved.
A successful “anticipator” function must predict either: 1) that the demand power is likely to remain higher than a motive power threshold, but well within the motor and battery capacities so that the engine will be started in as seamless a manner as possible; or 2) that the demand power is likely to exceed the motor and battery capacities within a very short time, and the engine should be started quickly in a “kick-down” fashion. In the latter case, sufficient motor torque must be held in reserve to compensate for the sudden load of the slewing engine.